1. Field of the Invention
The present invention relates to an acoustic gas analyzer.
2. Description of the Prior Art
In medical and clinical settings it is useful to be able to measure accurately the composition of respiratory (inspiration and/or expiration) gases or changes therein since such measurements can provide, for example, valuable information on patient metabolic conditions. This is particularly the case during the provision of mechanical respiratory aid to a patient where knowledge of the relative and absolute amounts of oxygen and carbon dioxide within the expiration gas may be used to provide information on the metabolization of oxygen as well as respiratory function. Moreover, knowledge of the oxygen/nitrogen ratio in an inspiration gas is useful for controlling or monitoring the provision of respiratory aid using a mechanical breathing aid such as a ventilator, respirator or anesthetic machine.
Gas analyzers are known, for example from WO 92/03724 and from U.S. Pat. No. 5,247,826, for acoustically analyzing the ratios of a mixture of gases comprising two known gases, such as the oxygen/nitrogen ratio in a breathing gas to be supplied to a patient by a mechanical breathing aid. The oxygen concentration or changes therein can then be determined. The known analyzers utilize the physical phenomenon that acoustic waves travel with different velocities through different gases. It is well known that the acoustic velocity, Vg, within a gas mixture can be described by an equation of the form:Vg=√{square root over (CP*R*Tg/M*Cv)}  (1)
CP and Cv are the specific heat capacities of the gas mixture at constant pressure and volume respectively; M is the molecular weight of the gas mixture; R is the universal gas constant; and Tg is its absolute temperature. Thus for a gas mixture at a known temperature, Tg, the acoustic velocity, Vg, in the mixture can be used to provide a measure of the relative concentrations of the constituents of the gas.
In general, the known acoustic gas analyzer has an ultrasonic velocity meter with a transducer arrangement that to transmits ultrasound energy along an acoustic path through a gas mixture to be analyzed within a measurement cell or a section of a flow conduit containing the flowing gas mixture and to receive the transmitted energy, a temperature probe disposed to monitor the gas temperature at a point within the cell or section and a calculator for calculating the acoustic velocity Vg of the ultrasound from signals received from the velocity meter and for employing this velocity together with the temperature reading from the probe, in a determination of compositional information for the analyzed gas mixture based on equation (1) above.
In order to determine the instantaneous composition of the gas mixture it is therefore necessary to know simultaneously the temperature of the gas and the acoustic velocity in the gas. It is often the case, however, that the time constant of the temperature probe is long compared with that of the ultrasonic transducer arrangement. The temperature probe may then provide a time-delayed picture of the actual temperature within the gas. This is particularly true where the gas undergoes temperature changes that are more rapid than the time constant of the temperature probe. A simultaneous measurement of the acoustic velocity and the temperature therefore can produce a small temperature error which will result in incorrect compositional information being determined. This can be a problem when the analyzer is employed in the determination of respiratory gas compositional information, since rapid gas pressure changes that normally occur within a ventilator system will result in temperature changes that may be faster than the time constant of the temperature probe.